Gravity dependent pedicle screw tap hole guide and data processing device

ABSTRACT

A gravity dependent pedicle screw tap hole guide comprises a guide shaft maintainable parallel to a drill bit during the drilling of a pedicle screw tap hole; an accelerometer associated with a reference direction and responsive to gravity to determine an angular difference between an acting direction of gravity and the reference direction; and a mounting attaching the accelerometer to the shaft and establishing a positional relationship between the reference direction and the longitudinal axis. The accelerometer is preferably in communication with a data processing device or system, which data processing device is able to use signals from the accelerometer to perform one or more actions, such as, for example, displaying the angular difference.

FIELD OF THE INVENTION

This application is a continuation application of U.S. patentapplication Ser. No. 10/452,394 entitled “Gravity Dependent PedicleScrew Tap Hole Guide and Data Processing Device” filed Jun. 2, 2003.This invention relates generally to devices and methods for insertingpedicle screws into the spine, and more specifically to devices andmethods for accurately establishing a pedicle screw tap hole drillingtrajectory.

BACKGROUND OF THE INVENTION

The bones and connective tissue of an adult human spinal column consistof more than 20 discrete bones coupled sequentially to one another by atri-joint complex which consist of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than 20 bones are anatomically categorized as being membersof one of four classifications: cervical, thoracic, lumbar, or sacral.The cervical portion of the spine, which comprises the top of the spine,up to the base of the skull, includes the first 7 vertebrae. Theintermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic andlumbar spine.

The spinal column of bones is highly complex in that it includes thesemore than 20 bones coupled to one another, housing and protectingcritical elements of the nervous system having innumerable peripheralnerves and circulatory bodies in close proximity. In spite of thesecomplexities, the spine is a highly flexible structure, capable of ahigh degree of curvature and twist in nearly every direction. Genetic ordevelopmental irregularities, trauma, chronic stress, tumors, anddisease, however, can result in spinal pathologies which either limitthis range of motion, or which threaten the critical elements of thenervous system housed within the spinal column. A variety of systemshave been disclosed in the art that achieve this immobilization byimplanting artificial assemblies in or on the spinal column. Theseassemblies may be classified as anterior, posterior, or lateralimplants. As the classifications suggest, lateral and anteriorassemblies are coupled to the anterior portion of the spine, which isthe sequence of vertebral bodies. Posterior implants generally comprisepairs of rods, which are aligned along the axis along which the bonesare to be disposed, and which are then attached to the spinal column byeither hooks which couple to the lamina or attach to the transverseprocesses, or by screws which are inserted through the pedicles.

The pedicles are the strongest parts of the vertebrae and thereforeprovide a secure foundation for the screws to which the rods are to beattached. In order to obtain the most secure anchor for the pediclescrews, it is essential that the screws be threaded in alignment withthe pedicle axis and not be allowed to deviate therefrom. Misalignmentof the pedicle screws during insertion can cause the screw body or itsthreads to break through the vertebral cortex and be in danger ofstriking surrounding nerve roots. A variety of undesirable symptoms caneasily arise when the screws make contact with nerves after breakingoutside the pedicle cortex, including dropped foot, neurologicallesions, sensory deficits, or pain.

Known surgical procedures to avoid misalignment of the pedicle screwsinvolve recognizing landmarks along the spinal column for purposes oflocating optimal tap hole entry points, approximating tap holetrajectories, and estimating proper tap hole depth. Some surgeons use aKocher clamp applied to the vertebral bone for a reference mark and/orview radiographs or other medical images to better understand relativepositions of the patient's anatomy. X-ray exposures and/or fluoroscopycan sometimes be used to monitor the advancement of a pedicle screwsthrough the vertebra. Unfortunately, these procedures are subject tosurgeon visual approximation errors, and anatomical landmarks aredifferent for each patient. Further, prolonged radiation exposure to apatient is undesirable. U.S. Pat. No. 4,907,577 (Mar. 13, 1990)discloses a jig that is described therein as providing a safe route fordrilling pedicle screw tap holes, by identifying a precise location fordrilling to prevent deviation from the drilling direction so as toprevent injury during surgery to the nerve root or spinal cord. However,the jig has a variety of moving parts that must be adjusted andmonitored simultaneously during the adjustments, making operation of thejig difficult and time consuming. Further, operation of the jig mustoccur during surgery, as it must be held adjacent the vertebral body todetermine the proper adjustment settings. Finally, adjustment of the jigto the proper settings requires precise visual approximation by thesurgeon, an activity that should be minimized to ensure that amisaligned trajectory is not established in place of a safe one.

More technologically advanced systems such as the StealthStation™Treatment Guidance System, the FluoroNav™ Virtual Fluoroscopy System(both available from Medtronic Sofamor Danek), and related systems, seekto overcome the need for surgeons to approximate landmarks, angles, andtrajectories, by assisting the surgeons in determining proper tap holestarting points, trajectories, and depths. However, these systems areextremely expensive, require significant training, are cumbersome inoperation, are difficult to maintain, and are not cost effective formany hospitals.

U.S. Pat. Nos. 5,474,558 (Dec. 12, 1995) and 5,196,015 (Mar. 23, 1993)propose a procedure in which a screw opening is started in part of askeletal region, e.g., a pedicle of a lumbar vertebra, and an electricpotential of a certain magnitude is applied to the inner surface of theopening while the patient is observed for nervous reactions such as legtwitching. The opening continues to be formed while the electricpotential is applied until a desired hole depth is obtained in theabsence of nervous reaction to the potential. The direction in which thescrew opening is being formed is changed to a direction other than thelast direction, after observing patient reactions to the electricpotential when the screw opening was being formed in the last direction.Unfortunately, this procedure is inherently reactive rather thanproactive, in that the surgeon becomes aware of the misalignment afterthe patient exhibits a nervous reaction, and by that time the misalignedhole has been drilled.

Therefore, there is a need for a simple device that eases thedifficulties associated with safely placing pedicle screws.Specifically, there is a need for such a device that assists a surgeonin making more accurate the surgeon's assessment of the proper insertiontrajectory of the pedicle screw. Further, there is a need for such adevice that does not require the surgeon to rely on visualapproximations. In addition, there is a need for such a device thatproactively determines the desirable drilling trajectory rather thanreactively informing the surgeon when an improper trajectory has beenused.

SUMMARY OF THE INVENTION

The needs identified above and other needs in the art are achieved bythe present invention that provides a gravity dependent pedicle screwtap hole guide and methods of use thereof.

One embodiment of a gravity dependent pedicle screw tap hole guide ofthe present invention has a shaft with a proximal end, a distal end, alongitudinal axis, and a fluid chamber attached to the shaft. A bubblein the fluid chamber indicates whether or not the chamber is leveland/or to what degree it is not level. The translucent wall of thechamber has a reference mark positioned so that that when the bubble iscentered under the reference mark, the longitudinal axis of the shaft isparallel to the acting direction of gravity. The wall also has a gridthat, when the bubble is not centered under the reference mark,indicates an angular difference (preferably in two perpendicular planes)between the longitudinal axis of the shaft and the acting direction ofgravity. Preferably, the longitudinal axis of the shaft extendsperpendicular to a plane in which a platform holding the chamberextends. The chamber is preferably a hemispherical enclosure with acentral axis that is parallel to the longitudinal axis of the shaft.

In operation of this embodiment, the surgeon first exposes a vertebralbone and applies a Kocher clamp to the spinous process in a verticalposition (where the longitudinal axis of the clamp is parallel to theacting direction of gravity) to his best visual approximation.Preferably, the guide of this embodiment is used here to make thevertical placement more accurate, by holding the shaft parallel to thelongitudinal axis of the Kocher clamp while manipulating the shaft withthe Kocher clamp so that when the bubble is centered under the referencemark, the surgeon knows that the Kocher clamp is in a vertical position.

Next, a lateral radiograph is taken and used to approximate thecephalad-caudad declination of the pedicle of interest and the medialangulation of the pedicle is determined from preoperative transaxial MRIand/or CAT scan images. The surgeon then positions the distal end of theshaft against the exposed vertebral bone in the vicinity of the base ofthe superior articular process and the base and middle of the transverseprocess (referred to herein as the “preferred tap hole entry point”),and angulates the shaft until the angular difference between thelongitudinal axis of the shaft and the acting direction of gravitymatches the determined cephalad-caudad declination (in thecephalad-caudad plane), and the medial angulation (in the medial plane).During this angulation, the surgeon can view the bubble's positionrelative to the grid lines, to know when and in what directionadditional angulational adjustment of the shaft is necessary to bringthe shaft to the desired position.

Once the shaft has been placed in the desired position, the surgeon canbe confident that drilling into the vertebral bone along the trajectoryestablished by the longitudinal axis of the shaft in the desiredposition will result in a pedicle screw tap hole that is formed tomaximize the stability of a pedicle screw subsequently screwedthereinto. The shaft can be hollow to accommodate a drill bit for thispurpose, or, if the shaft is not hollow, the distal end of the drill bitcan be placed against the preferred tap hole entry point of the exposedvertebral bone, and the shaft can be held parallel to the longitudinalaxis of the drill bit so that the shaft and the drill bit can beangulated in parallel together until the guide indicates the drill bitis at the desired angle.

Another embodiment of a gravity dependent pedicle screw tap hole guideof the present invention also has a shaft with an attached fluid chamberhousing a level-indicating bubble that rests under a reference mark whenthe chamber is level. The chamber is movably attached to the shaft andthereby positionable relative to the shaft. Specifically, the degree ofperpendicularity of the longitudinal axis of the shaft relative to aplane defined by the chamber can be varied in at least two planes. Inthis regard, the movable attachment of the chamber to the shaft isachieved by two rotatable mountings, the first being between the shaftand the second rotatable mounting, and the second being between thefirst rotatable mounting and the chamber. The first rotatable mountingrotates about an axis extending perpendicular to the longitudinal axisof the shaft, and the second rotatable mounting rotates about an axisextending perpendicular to the plane defined by the chamber. Each of therotatable mountings can be secured at any position to which it can berotated. When each rotatable mounting is in its zero position, the planeof the chamber is perpendicular to the longitudinal axis of the shaftand, accordingly, when the enclosure is oriented such that the bubble isunder the reference mark, the longitudinal axis of the shaft is parallelto the acting direction of gravity. Marks on the mountings preferablyindicate the relative angle of rotation of the rotatable mounting withrespect to the zero position, such that if either or both of therotatable mountings are placed in a rotated position, the user can readthe marks to determine the angular difference between the longitudinalaxis of the shaft and the plane defined by the chamber when the chamberis oriented so that the bubble is under the circle.

In operation of this embodiment, the surgeon proceeds as indicated abovewith regard to the first embodiment, but use of this embodiment to makethe Kocher clamp vertical placement more accurate is as follows: Therotatable mountings are placed in their respective zero positions, andthe shaft is held parallel to the longitudinal axis of the Kocher clampwhile being manipulated with the Kocher clamp until the bubble iscentered under the reference mark, at which time the surgeon knows thatthe Kocher clamp is in a vertical position.

After determining the cephalad-caudad declination and medial angulationof the pedicle of interest, the surgeon places the first rotatablemounting into a rotated position at an angular offset matching thecephalad-caudad declination, and places the second rotatable mountinginto a rotated position at an angular offset matching the medialangulation. During these rotations, the surgeon can view the marks toensure that the mountings are rotated to the desired angles. Then, thesurgeon positions the distal end of the shaft against the preferred taphole entry point, and angulates the shaft until the bubble is under thereference mark. The surgeon can then safely drill the tap hole asdesired along the trajectory established by the longitudinal axis of theshaft. Again, the shaft can be hollow and/or held in parallel to thedrill bit as the drill bit is angulated against the preferred tap holeentry point.

Yet another embodiment of a gravity dependent pedicle screw tap holeguide of the present invention is similar to the first embodimentdiscussed above, but uses an accelerometer instead of a fluid chamberhousing a level-indicating bubble. An accelerometer is known in the artas an electronic device that can determine its angular orientationrelative to the acting direction of gravity, and therefore can be usedto determine, for any device in fixed relation to the accelerometer, theangular orientation of that device relative to the acting direction ofgravity. The accelerometer can be connected to an analog or digitalreadout presenting the angular orientation of the accelerometer relativeto the acting direction of gravity. Preferably, the shaft is attached infixed relation to the accelerometer such that when the accelerometerindicates that there is no angular difference between the referencedirection recognized by the accelerometer and the acting direction ofgravity, the longitudinal axis of the shaft is parallel to the actingdirection of gravity. Accordingly, as the shaft is oriented freely inspace, the accelerometer indicates the angular difference (preferably intwo planes) between the longitudinal axis of the shaft and the actingdirection of gravity.

Operation of this embodiment proceeds as indicated with regard to thefirst embodiment, with the accelerometer (rather than thefluid-containing enclosure in the first embodiment) indicating when theshaft is in the desired position, that is, when the angular differencebetween the longitudinal axis of the shaft and the acting direction ofgravity matches the cephalad-caudad declination (in the cephalad-caudadplane) and medial angulation (in the medial plane) of the pedicle.

Still another embodiment of a gravity dependent pedicle screw tap holeguide of the present invention is similar to that of the secondembodiment described above, except that the fluid-containing enclosureof that embodiment is replaced with an accelerometer similar to theaccelerometer of the third embodiment described above. Accordingly, wheneach rotatable mounting is in its zero position, and the accelerometerreads level, the longitudinal axis of the shaft is parallel to theacting direction of gravity. And, accordingly, if either or both of therotatable mountings are placed in a rotated position, the user can, whenthe accelerometer is oriented level, read the marks to determine theangular difference between the longitudinal axis of the shaft and theacting direction of gravity.

Operation of this embodiment proceeds as indicated with regard to thesecond embodiment, with the accelerometer indicating when theaccelerometer is oriented level (and thus, if the rotatable mountingshave been rotated to match the cephalad-caudad declination and medialangulation of the pedicle, that the shaft is at the desired angulation).

In alternate embodiments, the accelerometers described above are incommunication with a data processing device or system. The dataprocessing device or system can be of any type known in the art.Preferably, the data processing system or device is able to do at leastone of the following: (1) obtain signals received from theaccelerometer, (2) interpret such signals, (3) determine desired data orinformation from such signals, (4) present such data or information toanother device or to a user (using any presentation manner known in theart, for example, but not limited to, visual, audible, or tactile), (5)use such data or information to direct or control another device or auser, and (6) perform any other desired action based on output from theaccelerometer. Preferably, the data processing system or device canalternatively or additionally send data or information to theaccelerometer in a medium that the accelerometer can interpret and/oract upon in a desired manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-c are side, top, and perspective views of an embodiment of agravity dependent pedicle screw tap hole guide of the present invention,the guide utilizing a fluid chamber housing a level-indicating bubble.

FIGS. 2 a-c are front, side, and top views of another embodiment of agravity dependent pedicle screw tap hole guide of the present invention,the guide utilizing a fluid chamber housing a level-indicating bubbleand rotatable mountings.

FIGS. 3 a-c are side, top, and perspective views of yet anotherembodiment of a gravity dependent pedicle screw tap hole guide of thepresent invention, the guide utilizing an accelerometer.

FIGS. 4 a-c are front, side, and top views of still another embodimentof a gravity dependent pedicle screw tap hole guide of the presentinvention, the guide utilizing an accelerometer and rotatable mountings.

FIGS. 5 a-c are side, top, and perspective views of still anotherembodiment of a gravity dependent pedicle screw tap hole guide of thepresent invention, the guide using an accelerometer in communicationwith a data processing device or system.

FIGS. 6 a-c are front, side, and top views of still another embodimentof a gravity dependent pedicle screw tap hole guide of the presentinvention, the guide utilizing rotatable mountings and an accelerometerin communication with a data processing device or system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions that follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope. Like numbers refer to similar features oflike elements throughout.

Referring now to FIGS. 1 a-c, an embodiment of a gravity dependentpedicle screw tap hole guide of the present invention is illustrated.The guide in this embodiment has a shaft 100 that has a proximal end 101and a distal end 102 and a longitudinal axis 105, and a fluid chamber110 attached to the shaft 100. The fluid chamber 110 is partially filledwith fluid 120, and the fluid 120 is contained within the chamber 110,such that a bubble 130 is present in the chamber 110. Because the gas inthe bubble 130 is lighter than the fluid in the chamber 110, the bubble130 floats in the chamber 110, seeking to travel in a direction oppositethe acting direction of gravity, but being prevented from leaving thechamber 110 because the chamber 110 is closed.

The chamber 110 has a wall 135 through which the bubble 130 is visible.The wall 135 has a reference mark 160 positioned so that that when thebubble 130 is centered under the reference mark 160, it is indicatedthat the longitudinal axis 105 of the shaft 100 is parallel to theacting direction of gravity.

Further, the translucent wall 135 has at least one relative mark (grid150) that can be read to determine the location of the center of thebubble 130 relative to the reference mark 160 when the bubble 130 is notcentered under the reference mark 160, the relative mark (grid 150)indicating an angular difference between the longitudinal axis 105 ofthe shaft 100 and the acting direction of gravity.

Preferably, as shown, the longitudinal axis 105 of the shaft 100 extendsin a direction perpendicular to a plane in which a platform 180laterally attached to the shaft 100 extends. The chamber 110 ispreferably a transparent hemispherical enclosure 110 having a centralaxis 170 (the axis 170 passing through the center top of the hemisphere110 and being perpendicular to the platform 180) is parallel to thelongitudinal axis 105 of the shaft 100.

Also preferably, the outer surface of the enclosure 110 is marked with aguide grid 150 formed by grid lines as shown. Grid lines in a first gridline set 140 are evenly spaced along the curved surface of the enclosure110 and extend in respective planes parallel to the longitudinal axis105 of the shaft 100. (Only one grid line of this set is marked 140merely for clarity in presentation of the figures; the reference numeral140 applies to the entire set of grid lines). Grid lines in a secondgrid line set 142 are evenly spaced along the curved surface of theenclosure 110 and extend in respective planes parallel to thelongitudinal axis 105 of the shaft 100 but perpendicular to the gridlines in the first set 140. (Only one grid line of this set is marked142 merely for clarity in presentation of the figures; the referencenumeral 142 applies to the entire set of grid lines). The central gridline of each set intersects with the other to define the reference mark160.

Accordingly, each grid line in the first set 140 indicates (when thebubble 130 is under the line) a respective angular difference betweenthe longitudinal axis 105 of the shaft 100 and the acting direction ofgravity in a first plane, and each grid line in the second set 142indicates (when the bubble is under the line) a respective angulardifference between the longitudinal axis 105 of the shaft 100 and theacting direction of gravity in a second plane perpendicular to the firstplane. The lines are preferably labeled to assist the surgeon inquantifying the angular difference. In this embodiment, grid lines inthe first set 140 are labeled in degrees, in reference to the firstplane, −40, −30, −20, −10, 0, 10, 20, 30, 40, respectively. Also in thisembodiment, grid lines in the second set 142 are labeled in degrees, inreference to the second plane, −40, −30, −20, −10, 0, 10, 20, 30, 40,respectively. It should be understood that other labeling, with greateror lesser angles, and/or greater or lesser increments, can also be used.

In operation of this embodiment, the surgeon first exposes the vertebralbone into which the pedicle screw is to be placed. Next, the surgeonapplies a clamp (e.g., a Kocher clamp) to the spinous process of theexposed vertebral bone, placing the Kocher clamp in a vertical position(parallel to the acting direction of gravity) to his best visualapproximation. Preferably, the gravity dependent pedicle screw tap holeguide of this embodiment is used at this point in the procedure to makemore accurate the surgeon's vertical placement of the Kocher clamp. Thatis, the shaft of the guide can be held parallel to the longitudinal axisof the Kocher clamp, manipulated with the Kocher clamp while beingmaintained in said parallel position, so that when the bubble 130 iscentered under the reference mark 160, the surgeon knows that the Kocherclamp is in the vertical position.

Once the Kocher clamp in attached to the spinous process in the verticalposition, a lateral radiograph is taken, and the cephalad-caudaddeclination of the pedicle of interest is determined by the surgeon tohis best visual approximation using the longitudinal axis of the Kocherclamp in the radiograph image as the “zero” axis. Also, the medialangulation of the pedicle is determined from preoperative transaxial MRIand/or CAT scan images. Angular measurement devices known in the art canbe used to make these angular assessments more accurate. Once thecephalad-caudad declination and the medial angulation have beendetermined, the surgeon positions the distal end 102 of the shaft 100against the exposed vertebral bone in the vicinity of the base of thesuperior articular process and the base and middle of the transverseprocess (referred to herein as the “preferred tap hole entry point”),and angulates the shaft 100 until the angular difference between thelongitudinal axis 105 of the shaft 100 and the acting direction ofgravity in the first plane matches the determined cephalad-caudaddeclination, and the angular difference between the longitudinal axis105 of the shaft 100 and the acting direction of gravity in the seconddirection matches the determined medial angulation. (It should beunderstood that alternatively, the device can be used with the gridlines in the set 140 begin use to match the medial angulation, and thegrid lines in the set 142 being used to match the cephalad-caudaddeclination.) During this angulation, the surgeon can view the positionof the bubble 130 under the guide grid 150, and particularly thebubble's position relative to the grid lines, to know when and in whatdirection additional angulational adjustment of the shaft 100 isnecessary to bring the shaft 100 closer to the desired position, andwhen the shaft 100 has reached the desired position.

Once the shaft 100 has been placed in the desired position, the surgeoncan be confident that drilling into the vertebral bone along thetrajectory established by the longitudinal axis of the shaft 100 in thedesire position will result in a pedicle screw tap hole that is formedto maximize the stability of a pedicle screw subsequently screwedthereinto. That is, the surgeon can be confident that the drilling isunlikely to result in penetration of the distal end of the drill bit toany outer surface of the vertebral bone, and is likely to result in thewalls of the tap hole being relatively uniformly thick at any givencross-section. Drilling into the vertebral bone along the trajectoryestablished by the longitudinal axis 105 of the shaft 100 in the desiredposition can be accomplished in that the shaft 100 can be hollow, asshown, with its internal diameter being sufficient to accommodate adrill bit suitable for drilling the tap hole, and with its length beingshorter than the exposed length of the drill bit (the amount of thedrill bit protruding from the drill) by an amount sufficient to allowthe drill bit to go into the bone to the clinically desired depth beforethe drill hits the proximal end of the shaft 100. The drill bit cantherefore be passed into the shaft 100, and can be rotated thereinduring the drilling, so that the tap hole is drilled along an extensionof the longitudinal axis of the shaft 100 at the desired angle.

Alternatively, the distal end of the drill bit can be placed against thepreferred tap hole entry point of the exposed vertebral bone, and theshaft 100 can be held parallel to the longitudinal axis of the drillbit. (This parallel holding can be accomplished, for example, by usingsuitable attachments or mountings for the shaft against the drill.) Thedrill bit and the shaft 100 can be angulated together (while beingmaintained in relative parallel positions) until the angular differencebetween the longitudinal axis 105 of the shaft 100 and the actingdirection of gravity in the first plane matches the determinedcephalad-caudad declination, and the angular difference between thelongitudinal axis 105 of the shaft 100 and the acting direction ofgravity in the second plane matches the determined medial angulation.(It should be understood that alternatively, the device can be used withthe grid lines in the set 140 begin use to match the medial angulation,and the grid lines in the set 142 being used to match thecephalad-caudad declination.) During this angulation, the surgeon canview the position of the bubble 130 under the guide grid 150, andparticularly the bubble's position relative to the grid lines, to knowwhen and in what direction additional angulational adjustment of thedrill bit (and parallel shaft 100) is necessary to bring the drill bitcloser to the desired position, and when the drill bit has reached thedesired position. Once the drill bit has been placed in the desiredposition, the surgeon can be confident that drilling into the vertebralbone along the trajectory established the longitudinal axis of the drillbit in the desired position will result in a pedicle screw tap hole thatis formed to maximize the stability of a pedicle screw subsequentlyscrewed thereinto.

Referring now to FIGS. 2 a-c, another embodiment of a gravity dependentpedicle screw tap hole guide of the present invention is illustrated.The guide in this embodiment has a shaft 200 that has proximal end 201and a distal end 202 and a longitudinal axis 205, and a fluid chamber210 attached to the shaft 200. The fluid chamber 210 is partially filledwith fluid 220, and the fluid 220 is contained within the chamber 210,such that a bubble 230 is present in the chamber 210. Because the gas inthe bubble 230 is lighter than the fluid in the chamber 210, the bubble230 floats in the chamber 210, seeking to travel in a direction oppositethe acting direction of gravity, but being prevented from leaving thechamber 210 because the chamber 210 is closed. Preferably, as shown, thechamber 210 defines a plane 215 that is perpendicular to the actingdirection of gravity when the chamber 210 is held level. The chamber 210has a translucent wall 235 through which the bubble 230 is visible. Thetranslucent wall 235 has a reference mark 260 positioned so that thatwhen the chamber 210 is held level, the bubble 230 is centered under thereference mark 260. The chamber 201 is movably attached to the shaft 200and thereby positionable relative to the shaft 200. Specifically, thedegree of perpendicularity of the longitudinal axis 205 of the shaft 200relative to the plane 215 defined by the chamber 210 can be varied in atleast two planes.

Preferably, as shown, a platform 282 is laterally attached to the shaft200. The chamber 210 is a transparent cylindrical enclosure 210 mountedon the platform 282, the bottom surface 215 of the chamber 210 definingthe plane 215. Also preferably, an upper surface 235 of the enclosure iscentrally marked with a circle 260. When the chamber 210 is oriented sothat the bottom surface 215 is held level, the bubble 230 is under thecircle 260.

Also preferably, the movable attachment of the chamber 210 to the shaft200 is achieved by two rotatable mountings 270, 280 between the chamber210 and the shaft 200. The first rotatable mounting 270 is between theshaft 200 and the second rotatable mounting 280. The second rotatable280 mounting is between the first rotatable mounting 270 and the chamber210. The first rotatable mounting 270 rotates about an axis 275extending perpendicular to the longitudinal axis 205 of the shaft 200,and the second rotatable mounting 280 rotates about an axis 285extending perpendicular to the plane 215 defined by the chamber 210.Each of the rotatable mountings 270, 280 can be secured at any positionto which it can be rotated. In this embodiment, the securing isaccomplished in each rotatable mounting by a set screw that when loose,permits rotation, and when tight, prevents rotation by pressing therelatively moving surfaces of the rotatable mounting against oneanother. Alternative or additional securing mechanisms can be providedwithin the scope of the present invention.

Also preferably, the angles of rotation that can be achieved by therotatable mountings are indicated by two sets 240, 242 of angle marksassociated respectively with each rotatable mounting 270, 280. Each sethas a zero mark, each zero mark indicating a zero position into whichthe associated rotatable mounting can be placed. When each rotatablemounting 270, 280 is in its zero position, the plane 215 of theenclosure 210 is perpendicular to the longitudinal axis 205 of the shaft200 and, accordingly, when the enclosure 210 is oriented such that thebubble 230 is under the circle 260, the longitudinal axis 205 of theshaft 200 is parallel to the acting direction of gravity.

Additional marks in the set preferably indicate the relative angle ofrotation of the rotatable mounting with respect to this zero position,such that if either or both of the rotatable mountings are placed in arotated position, the user can read the marks to determine the angulardifference between the longitudinal axis 205 of the shaft 200 and theplane 215 when the enclosure 210 is oriented so that the bubble 230 isunder the circle 260. Preferably, each set marks 10 degree increments,e.g., 40, −30, −20, −10, 0, 10, 20, 30, 40, with the first rotatablemounting marks indicating the angular difference in a first plane, andthe second rotatable mounting marks indicating the angular offset in asecond plane parallel to the first plane. It should be understood thatother labeling, with greater or lesser angles, and greater or lesserincrements, can also be used.

In operation of this embodiment, the surgeon proceeds as indicated abovewith regard to the first embodiment, applying a Kocher clamp in avertical position to the spinous process of the vertebral bone intowhich the pedicle screw is to be placed. The surgeon can again use hisbest visual approximation to apply the Kocher clamp vertically, or canpreferably use the gravity dependent pedicle screw tap hole guide ofthis embodiment to make the placement more accurate. That is, therotatable mountings 270, 280 of the guide can be placed in theirrespective zero positions, so that the plane 215 of the enclosure 210 isperpendicular to the longitudinal axis 205 of the shaft 200, and theshaft 200 can then be held parallel to the longitudinal axis of theKocher clamp, and manipulated with the Kocher clamp while beingmaintained in said parallel position similar to the use of the firstembodiment discussed above, so that when the bubble 230 is centeredunder the circle 260, the surgeon knows that the Kocher clamp is in avertical position.

Next, a lateral radiograph is taken and used to approximate thecephalad-caudad declination of the pedicle of interest, and the medialangulation of the pedicle is determined from preoperative transaxial MRIand/or CAT scan images. The surgeon then places the first rotatablemounting 270 into a rotated position at an angular orientation matchingthe cephalad-caudad declination, and places the second rotatablemounting 280 into a rotated position at an angular orientation matchingthe medial angulation. During these rotations, the surgeon can view therotatable mounting marks 240, 242 to ensure that the mountings 270, 280are rotated to the desired angles.

Then, the surgeon positions the distal end 202 of the shaft 200 againstthe preferred tap hole entry point of the exposed vertebral bone, andangulates the shaft 200 until the bubble 230 is under the circle 260.When the bubble 230 is under the circle 260, this indicates to thesurgeon that the angulation of the shaft 200 matches the angulation ofthe pedicle with respect to the vertical.

Once the shaft 200 has been placed in the desired position, the surgeoncan be confident that drilling into the vertebral bone along thetrajectory established by the longitudinal axis 205 of the shaft 200 inthe desired position will result in a pedicle screw tap hole that isformed to maximize the stability of a pedicle screw subsequently screwedthereinto. Drilling into the vertebral bone along an extension of thelongitudinal axis 205 of the shaft 200 can be accomplished in that theshaft 200 can be hollow, as discussed with regard to the firstembodiment, and the drill bit passed into and rotated in the shaft 200during the drilling. Alternatively, also as discussed with regard to thefirst embodiment, if a hollow shaft is not used, the shaft 200 can beheld parallel to the longitudinal axis of the drill bit, and the drillbit and the shaft 200 can be angulated together (while being maintainedin relative parallel positions) until the bubble 230 is under the circle260. When the bubble 230 is under the circle 260, this indicates to thesurgeon that the angular orientation of the shaft 200 (and therefore theangular orientation of the drill bit) matches the angular orientation ofthe pedicle with respect to the vertical.

Referring now to FIGS. 3 a-c, yet another embodiment of a gravitydependent pedicle screw tap hole guide of the present invention isillustrated. The guide in this embodiment has a shaft 300 that has aproximal end 301 and a distal end 302 and a longitudinal axis 305, andan accelerometer 310 attached to the shaft 300. The accelerometer 310 isan electronic device that can determine its angular orientation relativeto the acting direction of gravity, and therefore can be used todetermine, for any device in fixed relation to the accelerometer 310,the angular orientation of that device relative to the acting directionof gravity. Although a variety of accelerometers exist and can be usedwith the present invention, one example of an accelerometer that can beused with the present invention has as its central functional mechanisma computer chip that determines the angular orientation of a referencedirection relative to the acting direction of gravity, and further canbe connected to other electronic devices to provide relevant data inthat regard to such devices. A suitable accelerometer is sold by AnalogDevices, Inc. (Norwood, Mass.) as product number ADXL202. Accordingly,and preferably as shown, an analog or digital readout 320 incommunication with the accelerometer 310 is viewable to provide theangular orientation of the accelerometer 310 relative to the actingdirection of gravity.

Preferably, as shown, the shaft 300 is attached in fixed relation to theaccelerometer 310 such that when the accelerometer 310 indicates thatthere is no angular difference between the reference directionrecognized by the accelerometer 310 and the acting direction of gravity,the longitudinal axis 305 of the shaft 300 is parallel to the actingdirection of gravity. Accordingly, as the shaft 300 is oriented freelyin space, the accelerometer 310 indicates the angular difference(preferably in two dimensions) between the longitudinal axis 305 of theshaft 300 and the acting direction of gravity.

Operation of this embodiment proceeds as indicated with regard to thefirst embodiment, with the accelerometer 310 (rather than thefluid-containing enclosure of the first embodiment) indicating when theshaft 300 is in the desired position, that is, when the angulardifference between the longitudinal axis of the shaft 300 and the actingdirection of gravity matches the cephalad-caudad declination (in thefirst plane) and medial angulation (in the second plane) of the pedicle.

Referring now to FIGS. 4 a-c, still another embodiment of a gravitydependent pedicle screw tap hole guide of the present invention isillustrated. The guide in this embodiment is similar to that of thesecond embodiment described above, except that the fluid-containingenclosure 210 of that embodiment is replaced with an accelerometer 410similar to the accelerometer 310 described in the third embodimentdescribed above. Elements in this fourth embodiment that are similar tothose in the second embodiment are referenced with like numbers, but inthe four hundreds rather than the two hundreds. Accordingly, when eachrotatable mounting 470, 480 is in its zero position, and theaccelerometer 410 reads level, the longitudinal axis 405 of the shaft400 is parallel to the acting direction of gravity. And, accordingly, ifeither or both of the rotatable mountings are placed in a rotatedposition, the user can read the marks in the mark sets 440, 442 todetermine the angular difference between the longitudinal axis 405 ofthe shaft 400 and the acting direction of gravity when the accelerometer410 is oriented level.

Operation of this embodiment proceeds as indicated with regard to thesecond embodiment, with the accelerometer 410 indicating when theaccelerometer 410 is oriented level (and thus, if the rotatablemountings 470, 480 have been rotated to match the cephalad-caudaddeclination and medial angulation of the pedicle, that the shaft 400 isat the desired angulation).

Referring now to FIGS. 5 a-c, yet another embodiment of a gravitydependent pedicle screw tap hole guide of the present invention isillustrated. The guide in this embodiment is similar structurally andfunctionally to the third embodiment described above, with a differencein that the accelerometer is connected to a data processing device orsystem, which difference will be described in detail below. Moreparticularly, the illustrated embodiment is similar in all otherrespects to the third embodiment described above, and as such similarcomponents and features are numbered similarly, except in the 500srather than the 300s. The gravity dependent pedicle screw tap hole guideof this embodiment has a shaft 500 that has a proximal end 501 and adistal end 502 and a longitudinal axis 505, and an accelerometer 510attached to the shaft 500. The accelerometer 510 is similar to theaccelerometer 310 described in the third embodiment described above, butin this embodiment does not have a display (it should be understood thatin other variations of this embodiment, the accelerometer 510 can haveone or more displays of its own, and even its own data processingcapability separate from the data processing device or system 526described below). Preferably as shown, at least one input connection 522and at least one output connection 524 allow the accelerometer 510 tocommunicate with a data processing device or system 526. Either or bothof the input connection 522 and the output connection 524 can beaccomplished by any manner and/or via any device known in the art forallowing two objects to communicate, such as, for example, a wired orwireless connection. Furthermore, in some embodiments, it may bedesirable for only input connection(s) or only output connection(s) tobe present (i.e., in which signals and/or data and/or information issent only one way), and the present invention contemplates suchembodiments. Furthermore, it should be understood that although the dataprocessing device or system is shown as not being attached to the othercomponents of the tap hole guide, the present invention contemplatesembodiments in which the data processing device or system is mounted toone or more components of the tap hole guide. This would beadvantageous, for example, for embodiments where portability isdesirable.

Operation of this embodiment proceeds similarly as indicated with regardto the third embodiment, with the accelerometer 510 determining when theshaft 500 is in the desired position, that is, when the angulardifference between the longitudinal axis of the shaft 500 and the actingdirection of gravity matches the cephalad-caudad declination (in thefirst plane) and medial angulation (in the second plane) of the pedicle.As noted above, however, in addition, the accelerometer 510 is incommunication with the data processing device or system 526.

The data processing device or system 526 can be of any type known in theart, and is preferably able to do at least one of the following: (1)obtain signals received from the accelerometer 510, (2) interpret suchsignals, (3) determine desired data or information from such signals,(4) present such data or information to another device or to a user(using any presentation manner known in the art, for example, but notlimited to, visual, audible, or tactile), (5) use such data orinformation to direct or control another device or a user, and (6)perform any other desired action based on output from the accelerometer.For example, the data processing device or system 526 would preferablybe able to receive signals from the accelerometer 510 that are usefulfor determining the angular orientation of the accelerometer 510 (andthus that of the tap hole guide shaft 500) relative to the actingdirection of gravity, determine the angular orientation based on thosesignals, and then present the angular orientation (and/or other data orinformation related to that angular orientation) on a monitor screen forreading by the surgeon or other user).

Preferably, the data processing device or system 526 can alternativelyor additionally send data or information to the accelerometer 510 in amedium that the accelerometer 510 can interpret and/or act upon in adesired manner. For example, in some embodiments, it may be desirablefor the accelerometer 510 to be instructed as to the range of angles inwhich the tap hole guide should be permitted to travel, so that awarning can be presented by the accelerometer 510 (or another device)when the range is exceeded.

It should be understood that the present invention contemplates theintegration of the tap hole guide of this embodiment with other devicesand/or data, so that additional advantages can be achieved. Therefore,the present invention contemplates that the data processing device orsystem 526 is able to communicate with other devices, provide data tothose other devices, and/or process data obtained from those otherdevices, so that data from the other devices can be used in thedeterminations or presentations that the data processing device orsystem 526 makes. As one example, devices that are able to obtain orcontain data related to, and/or images of, the drilling site, preferablycan communicate with the data processing device or system 526 so thatthe system 526, using the drilling site data and the accelerometer data,can provide feedback to the surgeon or other user related to how closehe or she is to a desired drilling angle. Or, for another example,devices that can record data can be integrated with the data processingdevice or system 526 and with other equipment that has provided datarelated to the drilling site or the operation, so that after thesurgery, the surgeon or other person could view a computer-generatedplayback of the operation, complete with angulation data obtained fromthe accelerometer and information from the other equipment, for trainingor other purposes.

Preferably, as shown, the shaft 500 is attached in fixed relation to theaccelerometer 510 such that when the accelerometer 510 indicates thatthere is no angular difference between the reference directionrecognized by the accelerometer 510 and the acting direction of gravity,the longitudinal axis 505 of the shaft 500 is parallel to the actingdirection of gravity. Accordingly, as the shaft 500 is oriented freelyin space, the accelerometer 510 determines the angular difference(preferably in two dimensions) between the longitudinal axis 505 of theshaft 500 and the acting direction of gravity. Preferably, theaccelerometer 510 outputs signals via the output connection 524 to thedata processing device or system 526 for interpretation and/or any otherdesirable action as described above. Also preferably, the accelerometer510 receives signals from the data processing device or system 526 viathe input connection 522, which signals can be used by the accelerometer510 to achieve any desired result, such as, for example, calibration orreprogramming of the accelerometer 510. (It should be understood thatsome embodiments of the present invention use accelerometers that can bereprogrammed and/or recalibrated to achieve a variety of tasks suitableto a particular application. For example, such accelerometers couldinclude internal software or computer chips or “firmware” that can beupdated by the data processing device or system 526 to provide theaccelerometer with additional functionality or data. This would, forexample, obviate the need to purchase multiple types of accelerometersthat would need to be detached and re-attached to the pedicle screw taphole guide for different applications.)

Referring now to FIGS. 6 a-c, still another embodiment of a gravitydependent pedicle screw tap hole guide of the present invention isillustrated. The guide in this embodiment is similar structurally andfunctionally to the fourth embodiment described above, with a differencein that the accelerometer is connected to a data processing device orsystem as in the fifth embodiment described above. More particularly,the illustrated embodiment is similar in all other respects to thefourth embodiment described above, and as such similar components andfeatures are numbered similarly, except in the 600s rather than the400s. Accordingly, when each rotatable mounting 670, 680 is in its zeroposition, and the accelerometer 610 reads level, the longitudinal axis605 of the shaft 600 is parallel to the acting direction of gravity.And, accordingly, if either or both of the rotatable mountings areplaced in a rotated position, the user can read the marks in the marksets 640, 642 to determine the angular difference between thelongitudinal axis 605 of the shaft 600 and the acting direction ofgravity when the accelerometer 610 is oriented level.

The accelerometer 610 is similar to the accelerometer 410 described inthe fourth embodiment described above, but in this embodiment does nothave a display (it should be understood that in other variations of thisembodiment, the accelerometer 610 can have one or more displays of itsown, and even its own data processing capability separate from the dataprocessing device or system 626 described below). Preferably as shown,at least one input connection 622 and at least one output connection 624allow the accelerometer 610 to communicate with a data processing deviceor system 626. Either or both of the input connection 622 and the outputconnection 624 can be accomplished by any manner and/or via any deviceknown in the art for allowing two objects to communicate, such as, forexample, a wired or wireless connection. Furthermore, it someembodiments, it may desirable for only input connection(s) or onlyoutput connection(s) to be present (i.e., in which signals and/or dataand/or information is sent only one way), and the present inventioncontemplates such embodiments. Furthermore, it should be understood thatalthough the data processing device or system is shown as not beingattached to the other components of the tap hole guide, the presentinvention contemplates embodiments in which the data processing deviceor system is mounted to one or more components of the tap hole guide.This would be advantageous, for example, for embodiments whereportability is desirable.

Operation of this embodiment proceeds similarly as indicated with regardto the fourth embodiment, with the accelerometer 610 determining whenthe accelerometer 610 is oriented level (and thus, if the rotatablemountings 670, 680 have been rotated to match the cephalad-caudaddeclination and medial angulation of the pedicle, that the shaft 600 isat the desired angulation). As noted above, however, in addition, theaccelerometer 610 is in communication with the data processing device orsystem 626.

The data processing device or system 626 can be of any type known in theart, and is preferably able to do at least one of the following: (1)obtain signals received from the accelerometer 610, (2) interpret suchsignals, (3) determine desired data or information from such signals,(4) present such data or information to another device or to a user(using any presentation manner known in the art, for example, but notlimited to, visual, audible, or tactile), (5) use such data orinformation to direct or control another device or a user, and (6)perform any other desired action based on output from the accelerometer.For example, the data processing device or system 626 would preferablybe able to receive signals from the accelerometer 610 that are usefulfor determining the angular orientation of the accelerometer 610relative to the acting direction of gravity, determine the angularorientation based on those signals, and then present the angularorientation (and/or other data or information related to that angularorientation) on a monitor screen for reading by the surgeon or otheruser).

Preferably, the data processing device or system 626 can alternativelyor additionally send data or information to the accelerometer 610 in amedium that the accelerometer 610 can interpret and/or act upon in adesired manner. For example, in some embodiments, it may be desirablefor the accelerometer 610 to be instructed as to how far angularly fromparallel to the acting direction of gravity the accelerometer 610 shouldbe permitted to deviate (and thus how far angularly from the desiredcephalad-caudad declination and medial angulation the shaft 600 shouldbe permitted to deviate), so that a warning can be presented by theaccelerometer 610 (or another device) when the range is exceeded.

It should be understood that the present invention contemplates theintegration of the tap hole guide of this embodiment with other devicesand/or data, so that additional advantages can be achieved. Therefore,the present invention contemplates that the data processing device orsystem 626 is able to communicate with other devices, provide data tothose other devices, and/or process data obtained from those otherdevices, so that data from the other devices can be used in thedeterminations or presentations that the data processing device orsystem 626 makes. As one example, devices that are able to obtain orcontain data related to, and/or images of, the drilling site, preferablycan communicate with the data processing device or system 626 so thatthe system 626, using the drilling site data and the accelerometer data,can provide feedback to the surgeon or other user related to how dose heor she is to a desired drilling angle. Or, for another example, devicesthat can record data can be integrated with the data processing deviceor system 626 and with other equipment that has provided data related tothe drilling site or the operation, so that after the surgery, thesurgeon or other person could view a computer-generated playback of theoperation, complete with angulation data obtained from the accelerometerand information from the other equipment, for training or otherpurposes.

Preferably, in operation, once the surgeon has placed the firstrotatable mounting 670 into a rotated position at an angular orientationmatching the desired cephalad-caudad declination, and has placed thesecond rotatable mounting 680 into a rotated position at an angularorientation matching the desired medial angulation, he or she positionsthe distal end 602 of the shaft 600 against the preferred tap hole entrypoint of the exposed vertebral bone, and angulates the shaft 600 untilthe accelerometer 610 determines that the accelerometer 610 is level(the surgeon or other user is preferably able to view the accelerometer610 status preferably via a display on the data processing device orsystem 626). When the accelerometer 610 and/or data processing device orsystem 626 indicates that the accelerometer 610 is level, this indicatesto the surgeon that the angulation of the shaft 600 matches theangulation of the pedicle with respect to the vertical, and the surgeoncan proceed as indicated above with respect to the second and fourthembodiments.

Preferably, the accelerometer 610 outputs signals via the outputconnection 624 to the data processing device or system 626 forinterpretation and/or any other desirable action as described above.Also preferably, the accelerometer 610 receives signals from the dataprocessing device or system 626 via the input connection 622, whichsignals can be used by the accelerometer 610 to achieve any desiredresult, such as, for example, calibration or reprogramming of theaccelerometer 610. (It should be understood that some embodiments of thepresent invention use accelerometers that can be reprogrammed and/orrecalibrated to achieve a variety of tasks suitable to a particularapplication. For example, such accelerometers could include internalsoftware or computer chips or “firmware” that can be updated by the dataprocessing device or system 626 to provide the accelerometer withadditional functionality or data. This would, for example, obviate theneed to purchase multiple types of accelerometers that would need to bedetached and re-attached to the pedicle screw tap hole guide fordifferent applications.)

While there has been described and illustrated specific embodiments ofan intervertebral spacer device, it will be apparent to those skilled inthe art that variations and modifications are possible without deviatingfrom the broad spirit and principle of the present invention. Theinvention, therefore, shall not be limited to the specific embodimentsdiscussed herein.

1. A gravity dependent pedicle screw tap hole guide, comprising: a guideshaft having a proximal end, a distal end, and a longitudinal axis, theguide shaft being maintainable parallel to a drill bit during thedrilling of a pedicle screw tap hole with the drill bit; anaccelerometer associated with a reference direction and being responsiveto gravity to determine an angular difference between an actingdirection of gravity and the reference direction; and a mounting bywhich the accelerometer is attached to the guide shaft, the mountingestablishing a positional relationship between the reference directionand the longitudinal axis of the guide shaft.
 2. The gravity dependentpedicle screw tap hole guide of claim 1, wherein the guide shaft isfixed to the accelerometer by the mounting such that the longitudinalaxis of the guide shaft is parallel to the reference direction.
 3. Thegravity dependent pedicle screw tap hole guide of claim 1, wherein themounting is adjustable such that the longitudinal axis of the guideshaft is angulatable with respect to the reference direction.
 4. Thegravity dependent pedicle screw tap hole guide of claim 3, wherein themounting has at least one indicator that is viewable to determine anangular difference between the longitudinal axis of the guide shaft andthe reference direction.
 5. The gravity dependent pedicle screw tap holeguide of claim 3, wherein the mounting comprises at least one rotationalmounting between the guide shaft and the accelerometer, the rotationalmounting being engageable and disengageable at a plurality of positionsincluding a parallel position and a plurality of rotated positions, theparallel position being a position at which the longitudinal axis of theguide shaft is parallel to the reference direction, each of the rotatedpositions being a respective position at which the longitudinal axis ofthe guide shaft is rotated with respect to the reference direction. 6.The gravity dependent pedicle screw tap hole guide of claim 5, whereinthe mounting comprises first and second rotational mountings between theguide shaft and the accelerometer, the first rotational mountingproviding rotation of the longitudinal axis of the guide shaft relativeto the reference direction in a first plane, the second rotationalmounting providing rotation of the longitudinal axis of the guide shaftrelative to the reference direction in a second plane, the second planebeing perpendicular to the first plane, such that the parallel positionof the first rotational mounting is a position at which the longitudinalaxis of the guide shaft is parallel to the reference direction in thefirst plane, and the parallel position of the second rotational mountingis a position at which the longitudinal axis of the guide shaft isparallel to the reference direction in the second plane, each of therotated positions of the first rotational mounting being a respectiveposition at which the longitudinal axis of the guide shaft is rotatedwith respect to the reference direction in the first plane, and each ofthe rotated positions of the second rotational mounting being arespective position at which the longitudinal axis of the guide shaft isrotated with respect to the reference direction in the second plane. 7.The gravity dependent pedicle screw tap hole guide of claim 6, whereineach of the first and second rotational mountings has angle markersassociated therewith that are viewable to determine an angulardifference between the longitudinal axis of the guide shaft and thereference direction.
 8. The gravity dependent pedicle screw tap holeguide of claim 1, further comprising a data processing device incommunication with the accelerometer.
 9. The gravity dependent pediclescrew tap hole guide of claim 8, wherein the data processing devicepresents an indication of the angular difference.
 10. The gravitydependent pedicle screw tap hole guide of claim 8, wherein the dataprocessing device receives signals from the accelerometer, interpretsthe signals, determines data from the signals, and presents the data.11. The gravity dependent pedicle screw tap hole guide of claim 8,wherein the data processing device receives signals from theaccelerometer, interprets the signals, determines data from the signals,and directs a second device using the data.
 12. The gravity dependentpedicle screw tap hole guide of claim 8, wherein the data processingdevice uses signals received from the accelerometer to perform anaction.
 13. The gravity dependent pedicle screw tap hole guide of claim8, wherein the accelerometer uses signals received from the dataprocessing system to perform an action.
 14. A method of drilling apedicle screw tap hole, comprising: determining a trajectory angle as anangle of a pedicle in a reference plane relative to an acting directionof gravity; positioning at least one of a distal end of a drill bit anda distal end of a guide shaft of a gravity dependent pedicle screw taphole guide at a position adjacent the pedicle in a vicinity of a base ofa superior articular process of the pedicle and a base and a middle of atransverse process of the pedicle, the guide having the guide shaft andan accelerometer associated with a reference direction, theaccelerometer being responsive to gravity to determine an angulardifference between an acting direction of gravity and the referencedirection, the guide having a mounting by which the accelerometer isattached to the guide shaft, the mounting establishing a positionalrelationship between the reference direction and the longitudinal axisof the guide shaft; angulating the guide shaft about the distal end ofthe guide shaft until the gravity dependent pedicle screw tap hole guideindicates that an angle between the longitudinal axis of the guide shaftin the reference plane and the acting direction of gravity matches thetrajectory angle; and rotating the drill bit into the pedicle along atrajectory extending into the pedicle from the position at thetrajectory angle.
 15. The method of drilling a pedicle screw tap hole ofclaim 14, wherein the trajectory angle is a first trajectory angle andthe reference plane is a first reference plane, the method comprising:determining the first trajectory angle as the angle of the pedicle inthe first reference plane relative to the acting direction of gravity;determining a second trajectory angle as an angle of the pedicle in asecond reference plane relative to the acting direction of gravity;positioning at least one of the distal end of the drill bit and thedistal end of the guide shaft at the position; angulating the guideshaft about the distal end of the guide shaft until the gravitydependent pedicle screw tap hole guide indicates that the angle betweenthe longitudinal axis of the guide shaft in the first reference planeand the acting direction of gravity matches the first trajectory angle,and that an angle between the longitudinal axis of the guide shaft inthe second reference plane and the acting direction of gravity matchesthe second trajectory angle; and rotating the drill bit into the pediclealong a trajectory extending into the pedicle from the position at thefirst trajectory angle and the second trajectory angle.
 16. The methodof drilling a pedicle screw tap hole of claim 15, wherein the firstreference plane is a cephalad-caudad plane defined by a vertebral bodycomprising the pedicle, and the second reference plane is a medial planedefined by the vertebral body.
 17. The method of drilling a pediclescrew tap hole of claim 14, comprising: determining the trajectory angleas the angle of the pedicle in the reference plane relative to theacting direction of gravity; positioning the distal end of the guideshaft at the position; angulating the guide shaft about the distal endof the guide shaft until the gravity dependent pedicle screw tap holeguide indicates that the angle between the longitudinal axis of theguide shaft in the reference plane and the acting direction of gravitymatches the trajectory angle; positioning the drill bit coaxial with thelongitudinal axis of the guide shaft; and rotating the drill bit intothe pedicle along the trajectory extending into the pedicle from theposition at the trajectory angle.
 18. The method of drilling a pediclescrew tap hole of claim 14, comprising: determining the trajectory angleas the angle of the pedicle in the reference plane relative to theacting direction of gravity; positioning the distal end of the drill bitat the position; maintaining the longitudinal axis of the guide shaftparallel to the drill bit while angulating the guide shaft about thedistal end of the guide shaft until the gravity dependent pedicle screwtap hole guide indicates that the angle between the longitudinal axis ofthe guide shaft in the reference plane and the acting direction ofgravity matches the trajectory angle; and rotating the drill bit intothe pedicle along the trajectory extending into the pedicle from theposition at the trajectory angle.
 19. The method of drilling a pediclescrew tap hole of claim 14, wherein determining the trajectory anglecomprises: establishing a vertical orientation of a clamp having alongitudinal axis by maintaining the longitudinal axis of the guideshaft parallel to the longitudinal axis of the clamp while angulatingthe guide shaft about the distal end of the guide shaft until thegravity dependent pedicle screw tap hole guide indicates that thelongitudinal axis of the guide shaft is parallel to the acting directionof gravity; attaching the clamp in the vertical orientation to a spinousprocess of a vertebral body comprising the pedicle; generating an imageincluding the vertebral body while the clamp is attached; anddetermining from the image an angular difference between the orientationof the pedicle and the orientation of the clamp.
 20. The method ofdrilling a pedicle screw tap hole of claim 19, wherein determining thetrajectory angle further comprises obtaining at least one of an MRIimage of the vertebral body, a CAT image of the vertebral body, and aradiograph image of the vertebral body.